This invention relates to an improved method for sending binary data using a data communication system, which restricts the number of data characters which are acceptable for transmission such as electronic mail (E-mail).
E-mail systems are well known in the art for communicating data from one location to another location. Typically, data sent by electronic mail is in the form of 8-bit characters or bytes (a byte comprises 8-bits). It will be understood that 2.sup.8 or 256 different characters may be represented by these 8-bit characters. However, because particular ones of these 256 characters are reserved by the E-mail system for data communication protocol purposes, only the remaining ones of these 256 characters can be used for transmitting data, which are typically those which are present on the keyboard of a terminal, such as the keyboard of a personal computer. These data characters are typically referred to as displayable, printable or text characters. The number of data characters which can be transmitted by an E-mail system may be further restricted, since data communication lines frequently support only 7-bit codes, reserving the eighth bit as a parity check. In such case, only a total of 128 characters (data plus control characters) are available.
Since E-mail systems typically permit transmission of only a limited number of the 256 possible values provided by an 8-bit (one-byte) representation, as explained above, a problem is presented when it is desired to use E-mail for transmitting binary data, such as, for example, the contents of a file stored in the memory of a personal computer. Accordingly, when E-mail is to be used for such purpose, the binary file has to be modified so that its 256 8-bit data characters are converted into the particular subset of data characters which the E-mail system is able to deal with. Known methods for this purpose have the disadvantage of requiring the transmission of a significant number of extra bits and a relatively complicated implementation.